The present invention relates to a surface emitting semiconductor laser and a method of fabricating the same, and more particularly, it relates to a surface emitting semiconductor laser including a current confining region formed by ion implantation or selective oxidation and a method of fabricating this surface emitting semiconductor laser.
Vertical cavity surface emitting lasers are advantageous not only in obtaining a light beam with a circular section but also in two-dimensionally integrating plural emitting portions on a single substrate at a high density. Also, the vertical cavity surface emitting lasers can be operated with small power consumption and fabricated at a low cost. Owing to these advantages, the surface emitting semiconductor lasers have been regarded as a promising light source for the optical communications and optical information processing of the next generation, and have been variously examined and developed. Recently, the surface emitting laser diodes are realized in a variety of structures, including one having a very low threshold current of approximately 10 xcexcA (microampere) on the laboratory level and one commercially available at approximately 3000 yen.
The surface emitting semiconductor lasers are classified, depending upon their current confinement structures, into the following three types: Lasers including a simple mesa structure; lasers including a current confining layer formed by ion implantation; and lasers including a current confining layer formed by selectively oxidizing a semiconductor layer including Al. The lasers including a simple mesa structure have been utilized since the initial stage of the examination until today. The lasers including a current confining layer formed by ion implantation are widely used in commercially available semiconductor lasers. The lasers including a current confining layer formed by selectively oxidizing a semiconductor layer including Al are still now under research in laboratories. In view of electrical resistance and heat resistance, a laser with a planer structure including a current confining layer formed by ion implantation or selective oxidation, or a laser including a mesa structure with a very large area is advantageous to a laser including a simple mesa structure.
The surface emitting semiconductor lasers with a current confinement structure formed by ion implantation can be fabricated in various types of structures.
FIGS. 9A and 9B are schematic diagrams of conventional surface emitting semiconductor lasers 900 and 910, respectively described as a first conventional technique.
Each of the surface emitting semiconductor lasers 900 and 910 is formed on an n-type GaAs substrate 901, and has a multilayer structure for laser oscillation including an n-type lower mirror 902, an active region 904 and a p-type upper mirror 905. The lower mirror 902 is formed on the substrate 901, and the active region 904 is sandwiched between the lower mirror 902 and the upper mirror 905. The active region 904 comprises an active layer 903 of a strained quantum well including an In0.2Ga0.8As layer working as a well layer and a GaAs layer working as a barrier layer sandwiched between cladding layers of Al0.5Ga0.5As, and is designed so as to oscillate light with a wavelength of approximately 980 nm. Furthermore, a p-type electrode 906 is formed on the upper mirror 905. Also, an n-type electrode 907 is formed on the back surface of the n-type substrate 901, so that the light output of the laser can be taken out from the back surface of the substrate 901.
In the surface emitting semiconductor laser 900, an ion implanted region 908 formed by ion implantation is disposed in an area surrounding a given closed area within the upper mirror 905. On the other hand, in the surface emitting semiconductor laser 910, the ion implanted region 908 is formed so as to make the active region 904 be a closed area.
Now, the operation of the conventional surface emitting semiconductor lasers 900 and 910 will be described. Since the ion implanted region 908 is a relatively higher resistance area, a current injected into the active region 904 through the p-type electrode 906 and the n-type electrode 907 is confined by the ion implanted region 908. Accordingly, the current injected into the laser can be efficiently injected into the small closed area, resulting in largely decreasing a threshold current.
An example of the surface emitting semiconductor lasers including a current confining layer formed by selective oxidation is described in Applied Physics Letter, 66 (1995), pp. 3413-3415. FIG. 10 is a sectional view for schematically showing the structure of a second conventional surface emitting semiconductor laser 1000 described in this paper.
In the conventional surface emitting semiconductor laser 1000, an active layer 1020 and a p-type upper mirror 1030 are successively stacked on an n-type lower mirror 1010, and a mesa is formed through etching to expose the lower mirror 1010. Furthermore, a ring-shaped p-type electrode 1040 is formed on the top surface of the upper mirror 1030. The upper mirror 1030 is formed by stacking AlGaAs and GaAs, in which merely the lowermost AlGaAs is formed as an Al0.98Ga0.02As layer 1032 with a composition ratio of Al of 0.98 and the remaining portion is formed as an Al0.9Ga0.1As/GaAs mirror 1033 formed by alternately stacking Al0.9Ga0.1As layers with a composition ratio of Al of 0.9 and GaAs layers. By utilizing a difference (of approximately 15:1) in the oxidation rate between Al0.98Ga0.02As and Al0.9Ga0.1As, the Al0.98Ga0.02As layer 1032 alone is selectively oxidized from the side face of the mesa, thereby forming an AlxOy region 1031.
Next, the operation principle of the conventional surface emitting semiconductor laser 1000 will be described. Since the AlxOy region 1031 serves as an insulator, a current injected into the laser is confined by the AlxOy region 1031 so as to flow merely through the Al0.98Ga0.02As layer 1032, that is, a small area. Accordingly, the current can be efficiently confined, resulting in decreasing a threshold current. Moreover, owing to a difference in the refractive index between the Al0.98Ga0.02As layer 1032 and the AlxOy region 1031, light is confined in the lateral direction to some extent, which can further decrease the threshold current.
However, when the ion implanted region crosses the active layer in the first conventional technique, the threshold current can be increased because the active layer is damaged by the ion implantation. Also, when the ion implanted region is disposed within the upper mirror, although the ion implantation does not damage the active layer, the following problems can be caused: Since the concentration distribution of the implanted ion in the vertical direction has a spread (namely, the change in the ion concentration is not abrupt), the current confining region is unavoidably formed in a position away from the active layer. Accordingly, the current is spread in the lateral direction while flowing between the current confining region and the active layer, and hence, the current cannot be effectively confined. Also, since the current confining region has a large thickness in the vertical direction, the device resistance is inevitably increased.
On the other hand, in the second conventional laser, the semiconductor layer with a thickness of several tens nm is required to be oxidized from the side face of the mesa in the lateral direction by several tens xcexcm (micrometer). Since it is very difficult to control the oxidation rate and the shape to be oxidized, it is difficult to form the current confining region in a desired shape. When a necessary and minimum number of layers (one layer in FIG. 10) are to be oxidized so as not to increase the device resistance, it is necessary to form a hybrid mirror including two types of AlGaAs layers having different composition ratios of Al, which makes the fabrication difficult.
The objects of the present invention are providing (1) a surface emitting semiconductor laser in which an active layer is not damaged by formation of a current confining region through ion implantation and a method of fabricating this surface emitting semiconductor laser; and (2) a surface emitting semiconductor laser in which selective oxidation for forming a current confining region is highly accurately controlled and a method of fabricating this surface emitting semiconductor laser.
The surface emitting semiconductor laser of this invention comprises an active region including an active layer, and a lower mirror and an upper mirror sandwiching the active region, and a current confining region is disposed to surround a closed area below the active layer, and the current confining region is formed of a higher resistance layer obtained by implanting ions.
Alternatively, the surface emitting semiconductor laser of this invention comprises an active region including an active layer, and a lower mirror and an upper mirror sandwiching the active region, and at least one of the lower mirror and the upper mirror is formed of a semiconductor multilayer film obtained by alternately and repeatedly stacking a first layer and a second layer, and merely the first layer closest to the active region includes a closed area made of a semiconductor and an oxide area surrounding the closed area.
Furthermore, the method, of this invention, of fabricating a surface emitting semiconductor laser provided with an active region including an active layer, and a lower mirror and an upper mirror sandwiching the active region, comprises a first crystal growth step of forming a first multilayer structure excluding the active layer and including at least the lower mirror on a semiconductor substrate; an ion implantation step of selectively implanting ions into an outside area of a selected closed area on a top surface of the first multilayer structure, whereby forming a current confining region with a higher electric resistance around the closed area; and a second crystal growth step of forming a second multilayer structure including the active layer on the first multilayer structure after the ion implantation step.
Alternatively, the method of fabricating a surface emitting semiconductor laser provided with an active region including an active layer, and a lower mirror and an upper mirror sandwiching the active region, comprises a first crystal growth step of alternately and repeatedly stacking a first semiconductor layer including Al and a second semiconductor layer matching in lattice size with the first semiconductor layer, whereby forming, on a semiconductor substrate, a first multilayer structure having the second semiconductor layer as an uppermost layer; a step of forming a mask for defining a closed area on a top surface of the uppermost layer of the first multilayer structure; an oxidation step of forming a current confining region with a higher electric resistance by selectively oxidizing the first semiconductor layer below the uppermost layer from an area on a top surface of the uppermost layer of the first multilayer structure where the mask is not formed; and a second crystal growth step of forming a second multilayer structure on the first multilayer structure.